Increasing concerns about global climate change and energy security call for cost effective new approaches to reduce use of fossil fuels in cars and other vehicles. Recent domestic legislation, as well as the Kyoto protocol for greenhouse gas reduction, set challenging goals for reduction of CO2 emissions. For example, the California legislation phases in requirements for reducing CO2 generation by 30% by 2015. Other states may follow California in establishing lower emission goals. While new technologies, such as electric vehicles, are being pursued, cost effective approaches using currently available technology are needed to achieve the widespread use necessary to meet these aggressive goals for reduced fossil fuel consumption. Ethanol biofuel could play an important role in meeting these goals by enabling a substantial increase in the efficiency of gasoline engines.
One method of improving traditional gasoline engine efficiency is through the use of high compression ratio operation, particularly in conjunction with smaller sized engines. The aggressive turbocharging (or supercharging) of the engine provides increased boosting of naturally aspirated cylinder pressure. This pressure boosting allows a strongly turbocharged engine to match the maximum torque and power capability of a much larger engine. Thus, the engine may produce increased torque and power when needed. This downsized engine advantageously has higher fuel efficiency due to its low friction, especially at the loads used in typical urban driving.
Engine efficiency can also be increased by use of higher compression ratio. Compression ratio is defined as the ratio of the total volume of the cylinder when the piston is at the top of its stroke, as compared to its volume when the piston is at the bottom of its stroke. Like turbocharging, this technique serves to further increase the pressure of the gasoline/air mixture at the time of combustion.
However, the use of these techniques is limited by the problem of engine knock. Knock is the undesired rapid gasoline energy release due to autoignition of the end gas, and can damage the engine. Knock most often occurs at high values of torque, when the pressure and temperature of the gasoline/air mixture exceed certain levels. At these high temperature and pressure levels, the gasoline/air mixture becomes unstable, and therefore may combust in the absence of a spark.
Octane number represents the resistance of a fuel to autoignition. Thus, high octane gasoline (for example, 93 octane number vs. 87 octane number for regular gasoline) may be used to prevent knock and allow operation at higher maximum values of torque and power. Additionally, other changes to engine operation, such as modified valve timing may also help. However, these changes alone are insufficient to fully realize the benefits of turbocharging and higher compression ratio.
The use of higher octane fuels can reduce the problem of knocking. For example, ethanol is commonly added to gasoline. Ethanol has a blending octane number of roughly 110, and is attractive since it is a renewable energy source that can be obtained using biomass. Many gasoline mixtures currently available are about 10% ethanol by volume. However, this introduction of ethanol does little to affect the overall octane of the mixture. Mixtures containing higher percentages of ethanol, such as E85, suffer from other drawbacks. Specifically, ethanol is more expensive than gasoline, and is much more limited in its supply. Thus, it is unlikely that ethanol alone will replace gasoline as the fuel for automobiles and other vehicles. Other fuels, such as methanol, also have a higher blending octane number, such as 130, but suffer from the same drawbacks listed above.
It is known that the direct injection of an anti-knock fluid having alcohol content (such as ethanol or methanol) into the cylinder has a stabilizing effect on the gasoline/air mixture and reduces the possibility of knocking. In some embodiments, the anti-knock fluid may also include gasoline and/or water. FIG. 5 shows a representative boost system.
The boost system 100 includes a spark ignition engine 105, in communication with a manifold 110. The manifold 110 receives compressed air from turbocharger 120, and gasoline from gasoline tank 130. The gasoline and air are mixed in the manifold 110, and enter the engine 105, such as through port fuel injection. A second tank 140 is used to hold anti-knock fluid, which enters the engine 105 through direct injection. Additionally, the boost system 100 includes a knock sensor 150, adapted to monitor the onset of knock. The system also includes a boost system controller 160. The boost system controller receives an input from the knock sensor 150, and based on this input, controls the release of anti-knock fluid from the second tank 140 and the release of gasoline from the gasoline tank 130. In some embodiments, the boost system controller 160 utilizes open loop control to determine the amount of gasoline and anti-knock fluid to inject into the engine 105. In another embodiment, a closed loop algorithm is used to determine the amount of anti-knock fluid, based on the knock sensor 150, and such parameters as RPM and torque.
Ethanol has a high fuel octane number (a blending octane number of 110). Moreover, appropriate direct injection of ethanol, or other alcohol-containing anti-knock fluids, can provide an even larger additional knock suppression effect due to the substantial air charge cooling resulting from its high heat of vaporization. Calculations indicate that by increasing the fraction of the fuel provided by ethanol up to 100 percent when needed at high values of torque, an engine could operate without knock at more than twice the torque and power levels that would otherwise be possible. The level of knock suppression can be greater than that of fuel with an octane rating of 130 octane numbers injected into the engine intake. The large increase in knock resistance and allowed inlet manifold pressure can make possible a factor of 2 decrease in engine size (e.g. a 4 cylinder engine instead of an 8 cylinder engine) along with a significant increase in compression ratio (for example, from 10 to 12). This type of operation could provide an increase in efficiency of 30% or more. The combination of direct injection and a turbocharger with appropriate low rpm response provide the desired response capability.
Because of the limited supply of ethanol relative to gasoline and its higher cost, and to minimize the inconvenience to the operator of refueling a second fluid, it is desirable to minimize the amount of ethanol, or alcohol-based anti-knock fluid, that is required to meet the knock resistance requirement. By use of an optimized fuel management system, the required ethanol energy consumption over a drive cycle can be kept to less than 10% of the gasoline energy consumption. This low ratio of ethanol to gasoline consumption is achieved by using the direct ethanol injection only during high values of torque where knock suppression is required and by minimizing the ethanol/gasoline ratio at each point in the drive cycle. During the large fraction of the drive cycle where the torque and power are low, the engine would use only gasoline introduced into the engine by conventional port fueling. When knock suppression is needed at high torque, the fraction of directly injected ethanol is increased with increasing torque. In this way, the knock suppression benefit of a given amount of ethanol is optimized.
In one embodiment, an anti-knock fluid, such as an alcohol (such as ethanol or methanol) or alcohol blend with water and/or gasoline, is kept in a container separate from the main gasoline tank. As shown in FIG. 5, boost fluid from a small separate fuel tank is directly injected into the cylinders (in contrast to conventional port injection of gasoline into the manifold). The concept uses the direct fuel injector technology that is now being employed in production gasoline engine vehicles. The traditional path used by the gasoline is maintained, and is used to aspirate the gasoline prior to its injection into the cylinder. In situations where knocking may occur, such as high torque or towing, the anti-knock fluid is injected directly into the cylinder. The high heat of vaporization of the boost gas reduces the temperature of the gasoline/air mixture, thereby increasing its stability. In situations where knocking is not common, such as normal highway driving, the anti-knock fluid is not used. Thus, by limiting the use of the anti-knock fluid to only those situations where knocking is prevalent, the amount of anti-knock fluid used can be minimized.
By directly injecting the anti-knock fluid into the cylinder, knocking can be significantly reduced. This allows boost ratios of 2 to 3 and compression ratios in the 11 to 14 range. A fuel efficiency increase of 20%-30% relative to port fuel injected engines can be achieved using these parameters. Alcohol boosting can provide a means to obtain rapid penetration of high efficiency engine technology in cars and light duty trucks.
As noted above, the anti-knock fluid is kept in a separate container, and therefore may need to be replenished periodically. In some embodiments, it may be necessary for the operator to perform this function. If the need to replenish the anti-knock fluid is infrequent, such as is the case of windshield wiper fluid, the inconvenience is minimal. However, if the anti-knock fluid needs to be refilled often, this may present an unacceptable solution to consumers.
Therefore, it would be advantageous if a turbocharged spark-ignited engine could be injected with anti-knock fluids that eliminate the knocking issue, while remaining low cost and readily available. The engine could operate using gasoline, natural gas or any other fluid appropriate to spark-ignited engines. Furthermore, it would be beneficial if the use of this anti-knock fluid were minimized so as to reduce the frequency of replenishment.